In metal die casting, the liquid metal is pressed into a mold by means of a piston. In the hot chamber technique, the melting and holding crucible is part of the machine, and pressures of 200 N/cm2 (Newton per square centimeter) are used. In the cold-chamber technique, the liquid metal is filled into the machine and forced into the mold by means of a piston while pressures of 2,000 N/cm2 to 25,000 N/cm2 are usual. In accordance with the high arising pressures and the high temperatures of the molten metal, which enters into direct contact with the piston, a decisive question with regard to economy is how long the piston and in particular its front face will withstand the mechanical and thermal stresses. Essential factors in this regard are the lifetime of the sealing or piston rings that seal the piston against the surround cylinder wall. In addition it will be noted that any wear of the cylinder wall by the sealing rings should be avoided as far as possible as it is possible to replace the piston but any wear of the cylinder wall may entail an expensive overhaul or even the replacement of the casting tool.
One problem in the design of the sealing rings are the arising large temperature differences and variations. To accommodate the latter, the sealing rings require a moving space in the piston body. However, there is a risk that liquid metal may enter into these moving spaces through the gaps between the sealing rings and the piston body, thereby making it impossible for the sealing rings to contract on cooling. The result is an increasingly larger piston ring and an increased piston ring pressure on the cylinder wall, and thus increased wear. A common measure taken to reduce the need for such moving spaces is to cool the piston in the area where the sealing rings are arranged. However, the connection of the piston to the piston rod needs to be located behind the cooled section so that a great total piston length results. However, pistons of such a large size entail a high material usage and are expensive to manufacture. In this regard it should be noted that the size of the pistons corresponds to the size of the product. An example of the mass of a die casting product is one kilogram up to a ton. Larger units in the range of several tons or smaller parts are also possible, however.
Especially in large die casting machines, the relatively large cooled section of the piston requires a correspondingly high coolant flow that is often impossible to supply or not always available.
Such a piston is described in WO-A-03/074211. It is designed for a cold chamber die casting machine. The ingress of liquid metal into the expansion space of the sealing ring is prevented by making the circumference of the cover in front of this sealing ring large enough that a relatively narrow gap of a certain depth results. This gap, which is located in the cooled area of the piston, causes entering liquid metal to be strongly cooled and solidified in the gap already resulting in an additional sealing effect. However, this piston also suffers from the disadvantage that a considerable portion extending from the cover resp. the front face of the piston is being cooled so that all in all a great total length results and thus a high material usage for the piston.